1. Field of the Invention
The present invention relates to a mechanism and a method for driving an electrostatic actuator. In particular, the present invention relates to a mechanism for driving an electrostatic actuator which enables precise positioning and control of retention rigidity (ratio of positional restoration force to positional deviation) of the electrostatic actuator, and to a method for driving the electrostatic actuator.
2. Related Background Art
Various types of electrostatic actuators are disclosed in recent years. The electrostatic actuator is advantageous in (1) its high efficiency without causing loss, and (2) its simple structure constituted only of electrodes, being suitable for miniaturization. FIG. 9 shows an electrostatic actuator disclosed in Japanese Patent Publication No. 6-101938. In FIG. 9, stator 1001 comprises an insulative substrate, and driving electrodes 1002 formed on the surface thereof. Traveler 1003 constituted of a dielectric material is held with a clearance by a holding means (not shown in the drawing) to be movable in the directions shown by arrow marks a and b in the drawing. Driving circuit 1004 applies a voltage to driving electrodes 1002 provided on the surface of stator 1001. Driving circuit 1004 outputs three phases of driving voltages: "PHgr"A(1005), "PHgr"B(1006), and "PHgr"C(1007). Each of the phases is connected successively to driving electrodes 1002 as shown in FIG. 9. FIGS. 10A and 10B show the timing of voltage application by driving circuit 1004 to the electrodes. FIG. 10A shows application of pulse voltage of (0/+V) V of the respective phases (to the respective electrodes). FIG. 10B shows application of AC voltage of (+V/xe2x88x92V) V thereto. The application of the pulse voltage as the phases of "PHgr"A(1005), "PHgr"B(1006), and "PHgr"C(1007) as shown in FIG. 10A generates an electric field moving in the direction of the arrow mark a (the abscissa t representing the time). The moving electric field induces electric charge in traveler 1003 to exert a driving force to traveler 1003 to follow the moving electric field with a lag. Similarly, the application of pulsed voltage exerts a driving force to the traveler 1003. To drive the traveler in the reverse direction (the arrow b direction), the phase order of the applied voltage is reversed. This reversal can be conducted by exchanging two of the three voltage-phases.
FIG. 11 shows an electrostatic actuator disclosed in Japanese Patent Publication No. 7-112354. In FIG. 11, the parts 2001-2007 corresponds to the parts 1001-1007 in FIG. 9. FIG. 12 illustrates the traveler of the electrostatic actuator shown in FIG. 11. Traveler 2003 comprises driven electrodes 2008 which are arranged at regular intervals on the face confronting the stator and are kept at a fixed potential. This type of electrostatic actuator is driven by change of electrostatic capacity between the traveler and the stator, which is caused by displacement of the traveler. The voltage is applied to driving electrodes 2002 in such a pattern as shown in FIGS. 10A and 10B.
FIG. 13 shows an electrostatic actuator disclosed in Japanese Patent Application Laid-Open No. 6-261558, which floats or levitates the traveler above the stator by a squeeze film effect to remove the friction between the traveler and the stator. In FIG. 13, first piezo element 3011 is driven in accordance with the signal from first piezo element control circuit 3013, and second piezo element 3012 is driven in accordance with the signal from second piezo element control circuit 3014. In this electrostatic actuator, a squeeze film is formed between traveler 3003 and stator 3001 by driving first piezo element 3011 and second piezo element 3012 to support traveler 3003 by levitation without contact above stator 3001. Thereby, traveler 3003 is driven by application of a driving voltage to driving electrode 3002 in accordance with signals from electrostatic motor control circuit 3004 similarly as in the aforementioned electrostatic actuator.
However, problems below arise in precise positioning with an electrostatic actuator like those mentioned above. Firstly, in the electrostatic actuator which is driven stepwise by application of driving voltage in a waveform as shown in FIG. 10A, high resolution in positioning is not achievable, because the driving step depends on the pattern pitch of the driving electrode and the driving step cannot be made smaller than the pattern pitch, disadvantageously. On the other hand, in the electrostatic actuator which is driven by application of driving voltage in a A waveform as shown in FIG. 10B, a control parameter for the actuator is the frequency of driving voltage, namely the speed of actuation. Therefore, for precise positioning with this actuator, a feedback system with a position sensor of high resolution is necessary, which is generally expensive, resulting in high cost of the system. Further, the incorporation of the position sensor into the actuator makes difficult the miniaturization of the actuator. Furthermore, in a conventional electrostatic actuator, the standing position and retention rigidity xcexa of the traveler can not be set arbitrarily. In the present invention, the retention rigidity xcexa is a proportionality constant represented by the equation: xcexa=F/xcex94x (where F is the restoring force to restore the traveler to the prescribed standing position, and xcex94x is positional deviation of the traveler of the electrostatic actuator from the prescribed standing position). The retention rigidity K is desirably capable of being set arbitrarily since the optimum value thereof depends on the load and the driving conditions.
The present invention intends to provide a method of driving an electrostatic actuator which does not involve the aforementioned problems of conventional electrostatic actuator and can be provided at a low cost, can be miniaturized readily, and is capable of making the driving step pitch smaller than the pattern pitch of the driving electrode, and also to provide a mechanism of driving the electrostatic actuator.
The present invention intends also to provide a method of driving an electrostatic actuator which enables arbitrary setting of the retention rigidity of a positioned traveler, a mechanism of driving the electrostatic actuator, and an electrostatic actuator employing the method and the mechanism.
An embodiment of the method of driving an electrostatic actuator of the present invention which has a stator having driving electrodes in plural phases and a traveler relatively moved along the stator by application of a driving voltage to the driving electrodes, wherein the driving voltage applied to the driving electrodes is decided according to a voltage function employing as an argument a relative movement position of the traveler on the stator.
Another embodiment of the method of driving an electrostatic actuator of the present invention which has a stator having driving electrodes in plural phases and a traveler relatively moved along the stator by application of a driving voltage to the driving electrodes, wherein the driving voltage applied to the driving electrodes is decided according to a voltage function employing as an argument a retention rigidity with which the traveler is retained at a standing position on the stator.
A still another embodiment of the method of driving an electrostatic actuator of the present invention which has a stator having driving electrodes in plural phases and a traveler moved along the stator by application of a driving voltage to the driving electrodes, wherein the driving voltage applied to the driving electrodes is decided according to a voltage function employing as arguments a position of the traveler and a retention rigidity with which the traveler is retained at a standing position on the stator.
An embodiment of the electrostatic actuator mechanism of the present invention comprises a stator having driving electrodes in plural phases, a traveler relatively moved along the stator, a voltage applying means for applying a driving voltage to the driving electrodes, and a voltage function means for deciding the driving voltage applied to the driving electrodes according to a voltage function employing as an argument a relative movement position of the traveler on the stator.
Another embodiment of the electrostatic actuator mechanism of the present invention comprises a stator having driving electrodes in plural phases, a traveler relatively moved along the stator, a voltage applying means for applying a driving voltage to the driving electrodes, and a voltage function means for deciding the driving voltage applied to the driving electrodes according to a voltage function employing as an argument a retention rigidity with which the traveler is retained at a standing position on the stator.
A still another embodiment of the electrostatic actuator mechanism of the present invention comprises a stator having driving electrodes in plural phases, a traveler relatively moved along the stator, a voltage applying means for applying a driving voltage to the driving electrodes, and a voltage function means for deciding the driving voltage applied to the driving electrodes according to a voltage function employing as an argument a movement position of the traveler and a retention rigidity with which the traveler is retained at a standing position on the stator.